To insert a formula or function into a cell in Google Sheets:

- Click on the cell where you want to insert the formula or function.
- Type an equal sign (=) to start the formula.
- Type the name of the function or the formula.
- Type an opening parenthesis “(” to start the function arguments.
- Enter the arguments for the function or the formula. Each argument should be separated by a comma.
- Type a closing parenthesis “)” to end the function arguments.
- Press “Enter” or “Tab” on your keyboard to confirm the formula or function and display the result in the cell.

For example, to insert the SUM function to sum the values in cells A1, B1, and C1, you would type the following formula:

=SUM(A1, B1, C1)

You can also use the “Insert Function” dialog box to insert a function into a cell. To do this:

- Click on the cell where you want to insert the function.
- Click on the “fx” button to the left of the formula bar.
- In the “Insert Function” dialog box, type the name of the function or select it from the list of functions.
- Click on the “Insert” button to insert the function into the cell.
- Enter the arguments for the function as needed and click “OK” to confirm the function and display the result in the cell.

Keep in mind that you can use various functions in Google Sheets to perform calculations, look up data, manipulate text, and more. You can find a list of the available functions in the “Function Library” section of the “Insert Function” dialog box.